The invention relates to a fixing unit comprising a threaded terminal portion with one face having a hollow imprint able to receive a maneuvering tool of a complementary shape.
A fixing unit of this type is intended to be associated with a nut, in order to perform the assembly of at least two parts from one side of said assembly.
The invention finds a preferential application in the aircraft industry. However, it may be used in any other industrial field, notably when access to one of the sides of the parts to be assembled is difficult or impossible.
As notably illustrated in documents FR-A-2 383 350, U.S. Pat. No. 2,083,092 and U.S. Pat. No. 3,584,667, fixing units with a hollow head are known, such as screws, the head of which includes a hollow imprint, with a multilobate shape, provided for cooperating with a driving tool with a matching shape. The multilobate shape of the hollow imprint enables a larger tightening torque to be achieved than the one which would be achieved by using a polygonal imprint with equivalent characteristics. Indeed, the unfolded length of a polygonal contour with straight sides is smaller than the unfolded length of a contour with curved sides.
In these hollow head screws of the prior art, selection of the external diameter of the screw head is generally free and no stress is applied on said head after forming the imprint. Therefore, there is no risk of cracking the head of the screw around the imprint.
Moreover, as it is notably shown in document GB-A-893,356, it is known that the tightening torque obtained with a hollow head fixing unit increases with the engagement depth of the tool, i.e. with the depth of the imprint provided in the head of the fixing unit. This document also shows the increase in the tightening torque obtained when passing from a hexagonal imprint to an equivalent imprint with curved sides.
The use of fixing screws with one of its faces having a multilobe-shaped imprint is provided in U.S. Pat. No. 3,874,258.
Whether it be a screw or a nut, it may be seen that in all these known fixing units, the imprint for cooperating with the tool is always provided in a portion (screw head, etc.) with a bulging external shape. The reinforcement thereby obtained notably enables the tool to exert a large tightening torque without causing deformation of the material.
Fixing units are also known which include a threaded terminal portion with the end face including an imprint for receiving a driving tool. Such a headless unit, called a xe2x80x9ctubular screwxe2x80x9d or xe2x80x9chead-less screwxe2x80x9d, is generally used for performing the assembly of several parts. As notably illustrated in documents U.S. Pat. No. 2,133,409, U.S. Pat. No. 2,532,815 and U.S. Pat. No. 4,480,513, the imprint used is then essentially of a hexagonal shape.
As illustrated by documents U.S. Pat. No. 4,957,401 and FR-A-1198 913, fixing units provided with a head without an imprint but having a threaded terminal portion with a hexagonal imprint are also known. Such units are for assembling parts accessible from only one side.
It is known that the fixing units including an imprint in their threaded portion have lesser mechanical strength because of the presence of this imprint. This lesser strength is notably apparent when the thread is formed by roll bending and rolling the material at the surface of the blank, according to the technique described in document FR-A-2 469 229. This technique consists of mounting the blank between two rolls, with threads, in order to form the thread by pushing back the material, notably by exerting radial stresses on the rotating blank.
As illustrated in FIG. 1 of the appended drawings, when this shaping technique for thread 2 is applied on a blank 1 including an imprint 3 in the portion intended for forming the thread, clinks 4 may appear in the angles of the imprint, whether the latter is of a hexagonal or other shape. These clinks 4 originate from the uncontrolled variation of the stiffness of the cross-section on which rolling stresses are applied.
In document FR-A-2 584 151, a solution to this problem is provided by making a multilobe-shaped imprint provided with an odd number of lobes, preferably equal to seven. More specifically, the imprint formed in the threaded end of the fixing unit includes a peripheral edge in the shape of a continuous curved line including an odd number of identical lobes, regularly distributed on this edge.
As explained in this document, the particular shape of the imprint provides the advantage of allowing the latter to be made by plastic deformation of the metal, simultaneously with the forging of the blank, before the thread is formed by roll bending and rolling of the material at the surface of the rough specimen, by limiting the risks of cracking in the annular area located around the peripheral edge of the imprint. This advantage may particularly be appreciated in the case of materials which are difficult to deform plastically, for which risks of clink formation are particularly high. It is thereby possible, with this multilobate imprint shape to increase the torque applied on the fixing unit in order to hold it and contribute to the blocking of the nut.
However, tests conducted by the applicant have shown that relatively comparable results may be obtained with a fixing unit, the imprint of which having an even number of lobes. These tests have also shown that the use of an multilobe-shaped imprint, having an odd or even number of lobes, was not sufficient, per se, for suppressing any risks of occurrence of cracks during the roll bending and rolling operation subsequently to the formation of the imprint. Therefore, the problem consisting of surely eliminating the cracks upon making the thread is not solved in a totally satisfactory way by the teachings of document FR-A-2 584 151.
The object of the invention is specifically a threaded fixing unit with a multilobe-shaped imprint, the original design of which is able to surely prevent crack formation in the metal, when the thread is made by rolling after obtaining the imprint by plastic deformation of the metal, during the forging of the blank.
According to the invention, this result is obtained by means of a fixing unit comprising a threaded terminal portion with an end face having a hollow imprint able to receive a maneuvering tool of said unit, said imprint including a peripheral edge in the shape of a continuous curved line including at least three lobes, distributed on said edge, characterized in that the distance between the peripheral edge of the imprint and an external peripheral surface of the threaded terminal portion varies between a maximum distance and a minimum distance, whereby the ratio between said maximum and minimum distances is between about 1.25 and about 1.75.
Tests performed by the applicant have actually shown that the ratio between the aforementioned maximum and minimum distances is an essential feature enabling the definition of the lobes of the imprint to be optimized in order to allow the subsequent formation of the thread without any risks of occurrence of clinks or cracks. Thus, the applicant established that only fixing units in which the imprint is made by observing a ratio between the aforementioned maximum and minimum distances between about 1.25 and about 1.75, were able to prevent formation of clinks or cracks.
Although it is possible to make a fixing unit with the imprint including lobes of different shapes, the lobes are preferably all identical and regularly distributed.
Advantageously, each of the lobes comprises two side faces forming between them an angle, the apex of which is substantially located on the external peripheral surface of the threaded terminal portion.
This angle is preferably between about 23xc2x0 and about 37xc2x0. This feature, combined with that relating to the ratio between the aforementioned maximum and minimum distances, increases the stiffness of the whole, which enables the crack strength to be increased during the rolling operation. Actually, an angle greater than 37xc2x0 promotes occurrence of clinks under the rolling stresses, as the angle then tends to xe2x80x9copen and closexe2x80x9d, alternately. Conversely, an angle less than 23xc2x0 tends to form xe2x80x9cnatural tipsxe2x80x9d of overstress, which is also favorable to the formation of clinks.
Preferably, the peripheral edge of the imprint comprises four to ten lobes.
According to a first preferred embodiment of the invention, the peripheral edge of the imprint comprises five lobes and the aforementioned angle is between 26xc2x0 and 32xc2x0.
According to a second preferred embodiment of the invention, the peripheral edge of the imprint comprises seven lobes and the aforementioned angle is between 25xc2x0 and 27xc2x0.
In the preferred embodiments of the invention, the ratio between the maximum and minimum distances is substantially equal to 1.5.
Although may types of metal may be used, the invention is particularly suitable for the case when the fixing unit is made of a titanium alloy or of a metal material which is difficult to deform plastically.